Fabrication of deep-etched distributed Bragg reflectors in AlGaInP/GaAs laser stuctures
Edwards, Gareth T.
This thesis describes work undertaken to understand the physics of dry-etching AlGaInP/GaAs laser structures in Cl-based plasmas to develop a process that allows the fabrication of high-aspect-ratio, sub-micron features. The effects of the main process parameters on the etch rate, selectivity and post-etch surface roughness were studied to understand how the interactions between the semiconductor materials and the plasma gives rise to different sidewall geometries. The behaviour of the two types of material in the presence of a Cl-based plasma was found to be very different due to the production of InCb when etching the AlGaInP layers which is absent when etching GaAs. This product is involatile and rate limiting in Cl-rich plasmas. Effective removal of the inhibiting InCb was shown to be necessary to selectively etch the AlGaInP layers anisotropically at a reasonably high etching rate. A narrow range of plasma conditions satisfied these requirements. These conditions provided the thermal- and ion-assisted desorption of the InCh such that the removal rate matches the formation rate. In contrast, GaAs was shown to etch anisotropically under a much wider range of conditions as the mechanism for anisotropic etching does not require an inhibitor. With an optimized set of etch parameters, etching of high-aspect-ratio, sub-micron features in AlGaInP/GaAs laser structures was demonstrated. Despite the difference in etch mechanisms of GaAs and AlGaInP and the use of a 2-step process, at the same Ar fraction vertical sidewall features were achieved in both materials when etching sub-micron gratings. Under conditions that provide smooth vertical sidewalls the etch rate was shown to be aspect-ratio dependent due to the depletion of neutral species at the base of the feature. A range of 1-D gratings were etched with lattice constants as short as 200 nm and aspect ratios as high as 20:1. To the best of my knowledge these are the deepest vertically etched structures on this scale in AlGaInP/GaAs. Using this process, a range of devices including laser cavities defined by dry-etched facets and Distributed Bragg Reflectors and an integrated emitter/detector device were fabricated to demonstrate the quality of the etch process.
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